Conditional compilation fails if enum variants use `from()`

[yawor]

Hi,
I've stumbled upon an error when I've tried to add conditional compilation to my quick_error defined Error enum. Here's an example:

quick_error! {
 #[derive(Debug)]
 pub enum Error {
 Io(err: std::io::Error) {
 cause(err)
 from()
 display("I/O error: {}", err)
 }
 #[cfg(feature="ssh2")]
 Ssh(err: ssh2::Error) {
 cause(err)
 from()
 display("SSH error: {}", err)
 }
 Net(err: std::net::AddrParseError) {
 cause(err)
 from()
 display("Network error: {}", err)
 }
 Zmq(err: zmq::Error) {
 cause(err)
 from()
 display("ZMQ error: {}", err)
 }
 Other(desc: String) {
 description(desc)
 from(desc: &str) -> (String::from(desc))
 from(err: Box<dyn std::error::Error>) -> (String::from(err.description()))
 }
 }
}

If I remove the from() from Ssh variant, then it compiles correctly. When I'm expanding the macro, I can see the #[cfg(feature="ssh2")] everywhere except for the

impl From<ssh2::Error> for Error {
 fn from(err: ssh2::Error) -> Error {
 Error::Ssh(err)
 }
}

[jmjoy]

I meet the same problem.